1. Field of the Invention
The present invention relates to an optical frequency controlling device for ultra-dense wavelength-division-multiplexed optical channels, and more particularly, to an optical frequency controlling device for ultra-dense wavelength-division-multiplexed optical channels in which center frequencies of optical channels are aligned.
2. Description of the Related Art
To increase the transfer capacity of a strand of optical fiber, the gaps between the optical channels of the wavelength-division-multiplexed optical system have been being reduced from 100 GHz to 50 GHz, 25 GHz or the lower. There is no noticeable standard for classification of ultra-dense optical communications, but if the gaps between optical channels are usually 25 GHz or less and the number of the optical channels is 100 or more, such a communication belongs to the ultra-dense wavelength-division-multiplexed optical communication. In bi-directional optical communication systems, the difference between the center frequencies of optical channels through which beams travels in opposite directions can be very small as 10 GHz. This case is regarded to belong to the ultra-dense wavelength-division multiplexed optical communication.
In the conventional wavelength-division-multiplexed optical communication system, a wavelength locker or a Fabry-Perot filter controls center frequencies. However, since such a method to use optical elements has generally an error of a few GHz, in the wavelength-division-multiplexed optical communication system having a very narrow optical channel gap, it causes a serious problem. To align the ultra-dense wavelength-division-multiplexed optical channel, NTT of Japan employed a method in which one light source is modulated with a frequency corresponding to the optical channel frequency gap. However, in this method, its signal-to-noise ratio is low for each optical channel so that it is difficult to utilize it actually. Recently, Fiberspace company of United States has developed an innovative wavelength locker that lowers its error to about 100 MHz. However, every optical channel requires the wavelength locker for itself so that the cost problem still remains. As a result, the solution for this field has not been made completely.
Today, the device in which microwave beat frequency signal generated when optical detection is performed for optical channels of different wavelengths is used as a control signal and the gap between optical frequencies of the optical channel sections is maintained to be uniform is suggested in the related art. Using this device, relative optical channel gaps between the nodes communicate with each other in the bi-directional optical communication can be controlled much more precisely than a general wavelength locker. This technology is disclosed in the following paper: Yong-Sang Ahn, Sang-Yuep Kim, Sang-Hoon Han, Jae-Seung Lee, Sang-Soo Lee, and Wan-Seok Seo, “Bidirectional DWDM transmission using a beat-frequency-locking method,” IEEE Photonics Technology Letters, vol. 13, pp. 899-901, 2001. However, since this technology is proper to keep the relative frequency between different optical channel sections to be stable, this technology is not proper to align light sources to be used in the general ultra-dense wavelength-division-multiplexed optical communication other than the bi-directional optical communication.
The technology in which two or three light sources are aligned using beat frequency signals has been studied. Usually, the difference between an actual frequency of a beat frequency signal and the desired frequency of the beat frequency signal is converted into electric signal and the optical channel gap or the channel spacing is stabilized using the converted electric signal. For example, in the U.S. Pat. No. 5,861,975, the technology in which the beat frequency signal is converted into the voltage signal proportional to the beat frequency and the converted voltage signal is used to align the optical channel. This is a suitable method to align a few light sources but it cannot be used when the numbers of optical channels and beat frequency signals are large.